WASHINGTON — Andy Pettitte became the first pitcher over 40 to start a game for the Yankees since Roger Clemens took the mound five years ago at 45.

Though Pettitte gave the Yankees just about everything they could want, he admitted to being “gassed” after seven innings and 95 pitches.

“I don’t know if it was from the heat, a day game, or having to go up there [to the plate] with a different routine,” Pettitte said of why the outing took so much out of him. “Maybe it’s being 40.”

The lefty said it with a laugh after hitting the milestone on Friday and then watching the Yankees battle through a 5-3, 14-inning win over the Nationals yesterday, but Pettitte is still attempting to defy time.

Pettitte gave up just two runs in seven innings and was in line to earn his fourth win when Cory Wade gave up a game-tying homer to Ian Desmond in the eighth. The only two runs Pettitte surrendered came on a broken-bat double by Jesus Flores in the second.

The veteran also dominated 19-year-old Bryce Harper, striking out the phenom three times before the center fielder lined out to center in the seventh — which turned out to be Pettitte’s last batter.

Pettitte said he hadn’t given much thought to his between-start regimen until he was asked about it yesterday, but he admitted it was something he might have to reconsider.

“I ran every day in Atlanta,” Pettitte said of the team’s recent trip down South. “It was hot and maybe I ran too much. … Those are things I have to evaluate.”

But he would rather not alter anything.

“I’m trying to act like I’m not 40 and I’m like I was a few years ago and do my routine and what I’ve always done because I’m such a creature of habit,” Pettitte said. “I throw two bullpens between starts. … I don’t think it’s gonna happen but if I continue to get tired, I’ll have to look at it.”

He threw 95 pitches in his previous outing, as well.

“The other day was a day game at home and I threw [38] pitches in the second inning and, I’m not going to lie to you, I didn’t think I would get out of the third inning because I so tired.”

Though he may tire a little more easily than he used to, the Yankees certainly will continue to take these results from Pettitte, who lowered his ERA to 2.77 in seven starts.